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The object of the present study was to test the hypotheses of possible relationships between the amplitude A of the oral temperature circadian rhythm and (a) tolerance to shift-work on the one hand, (b) the speed of adjustment on shift-work on the other hand. 29 oil refinery shift-workers (on a 3-4 day rotation system) volunteered for the study. Four groups were formed. (I) 6 young operators with no medical complaint; subjects’ age: 21 to 35 y: shift-work duration: 1 to 4 y. (II) 10 senior operators with no medical complaint; 44 to 57 y; shift-work duration: 15 to 32 y. (111) 6 senior operators with minor medical complaints; 46 to 56 y; shift-work duralion:22 to 31 y. (IV) 7 senior operators with major medical complaints (persisting fatigue during rest and days off; deep subjective deterioration of sleep; use of sleeping pills etc.): 30 to 56 y of age, shift-work duration: 9 to 29 y. Oral temperatures were self-measured and recorded every 4 h, at fixed limes, over a 3 to 4 week span in order to obtain for each subject a longitudinal limes series of about 100 data. In addition, characteristics of sleep and fatigue were self-rated daily on special sheets. Both conventional and cosinor methods were used for statistical analyses. A circadian rhythm is validated in the 4 groups without differences in mesor (24 h adjusted average) nor in acrophase (crest location in the 24 h scale). A fall in the amplitude A is observed in group IV of non-toleranl subjects (2A=0.46°C; from 0.34 to 0 58 with 95% confidence limits) in comparison with group I (2A = 0.74°C: from 0 58 to 0 90) and group 11 (2A =0 70°C; from 0.60 to 0.80). A negative correlation (r = 0.55; p<0.01) is found between A and the acrophase shift δϕ resulting from night-shifts: the larger the amplitude, the smaller the δϕ Thus, the hypotheses tested can be regarded as complementary: a subject tolerant to shift work over several years is likely: (1) to have a large amplitude of the temperature circadian rhythm and (2) to adjust slowly during the night-shift (small δϕ).